Necrotising myositis – Diagnostics – Overview of Information and Clinical Research

Diagnosing necrotizing myositis requires a combination of careful clinical evaluation, specialized blood tests, and tissue examination. Because this rare muscle disease can present with symptoms similar to other conditions, and because many physicians may not be familiar with its signs, accurate diagnosis often depends on multiple diagnostic approaches working together.

Introduction: Who Should Seek Diagnostic Testing

If you are experiencing muscle weakness that affects both sides of your body, particularly in the muscles closest to your torso such as your shoulders, hips, thighs, or upper arms, it is important to talk to your doctor about beginning the diagnostic process. Necrotizing myositis, also called immune-mediated necrotizing myopathy or IMNM, is a rare autoimmune muscle disease that requires prompt diagnosis and treatment to prevent long-term disability.^[1]

You should seek medical evaluation if you notice difficulty climbing stairs, standing up from a chair, lifting your arms over your head, or getting up after a fall. These seemingly simple activities become challenging when the proximal muscles—those nearest to the center of your body—are weakened by muscle cell death, which is what happens in necrotizing myositis. Some people may also experience difficulty swallowing, shortness of breath, or a chronic dry cough, though these symptoms are less common.^[1]

The onset of symptoms can happen relatively quickly, developing over days, weeks, or months, and typically occurs in adults between the ages of 40 and 60. However, children can also be affected, and in younger patients the disease may initially resemble a muscular dystrophy.^[1]^[2] Because necrotizing myositis is a rare disease, many physicians may not immediately recognize the signs and symptoms. If you are struggling to get an accurate diagnosis from your regular doctor, visiting a specialist—such as a rheumatologist or a physician at a myositis specialty center—can be very helpful.^[1]

⚠️ Important

Early diagnosis and treatment of necrotizing myositis is critical. Both anti-SRP and anti-HMGCR forms of the disease can cause severe weakness, and muscle atrophy with irreversible fatty replacement can happen soon after disease onset. This means that starting aggressive treatment early may provide the best chance to avoid long-term disability and preserve muscle function.

Classic Diagnostic Methods

Diagnosing necrotizing myositis involves several steps and different types of testing. Because the symptoms can resemble those of other muscle diseases, doctors use a combination of clinical evaluation, blood tests, electrical studies, and tissue examination to confirm the diagnosis and distinguish it from conditions like polymyositis—an inflammatory muscle disease that was often confused with necrotizing myositis before doctors learned to tell them apart.^[1]

Blood Tests

One of the first and most important blood tests your doctor will order is a measurement of creatine kinase, or CK. This is an enzyme that leaks out of damaged muscle cells into the bloodstream. In people with necrotizing myositis, CK levels are typically highly elevated—often much higher than in other forms of myositis. Some patients may have CK levels that are 10 to 100 times higher than normal, which is a strong indicator that significant muscle damage is occurring.^[2]^[5]

Blood tests can also detect specific autoantibodies—proteins produced by the immune system that mistakenly attack the body’s own tissues. In necrotizing myositis, two particular autoantibodies are closely associated with the disease: anti-SRP (anti-signal recognition particle) and anti-HMGCR (anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase). These are called myositis-specific autoantibodies and they help doctors not only confirm the diagnosis but also predict the disease course and choose the best treatment approach.^[1]^[3]

The presence of anti-SRP autoantibodies is often associated with more severe muscle weakness and a higher likelihood of difficulty swallowing. Some patients with anti-SRP may also develop inflammation in the lungs or heart. Anti-HMGCR autoantibodies, on the other hand, are sometimes found in people who have been exposed to cholesterol-lowering medications called statins, or to dietary sources like red yeast rice and oyster mushrooms, although in some cases no trigger can be identified.^[1]^[3]

However, not everyone with necrotizing myositis has these autoantibodies. Some patients test negative for both anti-SRP and anti-HMGCR, and these individuals are considered to have a third category called seronegative necrotizing myositis.^[1]^[5]

Electromyography (EMG)

Electromyography, often called EMG, is an electrical test used to check muscle strength and function. During this test, small electrodes are placed on or inserted into the muscles to measure their electrical activity. In people with necrotizing myositis, the EMG typically shows a pattern consistent with myopathy—a term that simply means muscle disease—but it does not specifically tell doctors what type of myopathy is present or what is causing it. The test is helpful in confirming that muscle weakness is due to a problem within the muscles themselves rather than a problem with the nerves.^[5]^[13]

Muscle Biopsy

The muscle biopsy is one of the most important diagnostic tests for necrotizing myositis. During a biopsy, a small sample of muscle tissue is removed—usually from the thigh or upper arm—and examined under a microscope. This allows doctors to see what is happening inside the muscle at a cellular level.^[1]

In necrotizing myositis, the muscle biopsy shows a characteristic pattern: there is significant necrosis, which means muscle cells are dying, but there is often very little inflammation compared to other types of myositis. This combination—prominent muscle cell death with minimal inflammatory cell infiltration—is what distinguishes necrotizing myositis from conditions like polymyositis, where inflammation is a major feature. The biopsy can also show other changes, such as regenerating muscle fibers trying to repair the damage.^[1]^[2]

Before necrotizing myositis was recognized as a separate disease, many patients with this pattern on their muscle biopsy were incorrectly diagnosed with polymyositis because the two conditions share similar clinical features such as proximal muscle weakness. Today, the combination of muscle biopsy findings and the presence of specific autoantibodies helps doctors make the correct diagnosis.^[1]

Imaging Studies

While not always necessary for diagnosis, imaging tests such as magnetic resonance imaging (MRI) can sometimes be helpful. MRI scans of the muscles can show areas of inflammation, damage, and—in more advanced cases—fatty replacement of muscle tissue. This fatty replacement is a sign of irreversible muscle damage and can occur soon after disease onset in both anti-SRP and anti-HMGCR myopathy, which is why early and aggressive treatment is so important.^[2]

Diagnostics for Clinical Trial Qualification

Clinical trials are research studies that test new treatments for diseases. For patients with necrotizing myositis, participating in a clinical trial may offer access to promising new therapies that are not yet widely available. However, to enroll in a clinical trial, patients must meet specific criteria, and certain diagnostic tests are used to determine whether someone is eligible.^[1]

Most clinical trials for necrotizing myositis require confirmation of the diagnosis through a muscle biopsy showing the characteristic pattern of muscle cell necrosis with minimal inflammation. Trials may also require documentation of specific autoantibodies—either anti-SRP or anti-HMGCR—measured through standardized blood tests. The presence or absence of these autoantibodies often determines which clinical trial a patient may be eligible for, since different treatments may work better for different subtypes of the disease.^[2]^[9]

In addition to the muscle biopsy and autoantibody testing, clinical trials typically require baseline measurements of disease activity and severity. This includes blood tests to measure creatine kinase levels, which serve as a marker of ongoing muscle damage. Physical function assessments, such as standardized muscle strength testing and questionnaires about the ability to perform daily activities, are also commonly used to track how well patients are functioning before treatment begins.^[9]

Some clinical trials may also require imaging studies, such as MRI scans of affected muscles, to document the extent of muscle damage and inflammation at the start of the study. These baseline images can then be compared to images taken later in the trial to see whether the experimental treatment is helping to reduce muscle inflammation or prevent further damage.^[2]

Depending on the focus of the clinical trial, additional tests may be needed. For example, if a trial is studying a treatment for patients who have lung involvement, pulmonary function tests—which measure how well the lungs are working—may be required. If the trial is looking at heart complications, an electrocardiogram (a test that measures the heart’s electrical activity) or an echocardiogram (an ultrasound of the heart) might be necessary.^[3]

It is important to remember that clinical trial enrollment criteria are designed to ensure patient safety and to gather reliable data about whether a new treatment works. Your doctor can help you understand whether you might be eligible for any ongoing clinical trials and what diagnostic tests would be needed to determine your eligibility.

Prognosis and Survival Rate

Prognosis

The outlook for people with necrotizing myositis varies considerably depending on several factors, including the subtype of the disease, the age at which symptoms begin, how quickly treatment is started, and how well the disease responds to therapy. Both anti-SRP and anti-HMGCR myopathy can cause severe and debilitating weakness, and younger age at disease onset is generally considered a poor prognostic factor—meaning that children and young adults with necrotizing myositis tend to have more severe disease that is harder to control.^[2]^[9]

One of the most concerning aspects of necrotizing myositis is that muscle atrophy and irreversible fatty replacement of muscle tissue can happen very early in the disease course. This means that even if treatment eventually brings the autoimmune process under control, some of the muscle damage that occurred early on may be permanent. For this reason, prompt initiation of aggressive immunosuppression—medications that suppress the overactive immune system—is often critical to preserving muscle function and avoiding long-term disability.^[2]^[9]

Patients with anti-HMGCR myopathy may have a somewhat better response to treatment, particularly with intravenous immunoglobulin (IVIG), and in some cases IVIG alone may be sufficient to control the disease. In contrast, patients with anti-SRP myopathy often have more severe muscle involvement and may require combination therapy with multiple immunosuppressive medications. Early use of rituximab, a medication that targets specific immune cells, is commonly favored for anti-SRP myopathy.^[2]^[9]

Many patients with necrotizing myositis require long-term treatment to keep the disease under control, and there is a high rate of relapse when medications are tapered or stopped. This means that ongoing monitoring and adjustments to treatment are often necessary. However, with appropriate and timely treatment, many people with necrotizing myositis can achieve significant improvement in muscle strength and function, and some are able to return to normal or near-normal activities.^[9]

Survival rate

While necrotizing myositis can be a serious and disabling condition, survival rates have improved as doctors have gained a better understanding of the disease and how to treat it. In the past, when anti-SRP myopathy was first being recognized, patients were sometimes told that the disease could be very resistant to treatment and that mortality rates might be high. However, with advances in diagnosis and treatment over the past several years, more and more people are living successfully with immune-mediated necrotizing myopathy for many years after diagnosis.^[14]

The most significant risks to life in necrotizing myositis typically come from complications rather than the muscle weakness itself. For example, severe weakness of the throat muscles can lead to difficulty swallowing, which increases the risk of choking or aspiration—when food or liquid enters the lungs instead of the stomach. Aspiration can cause pneumonia, a serious lung infection. Similarly, if the respiratory muscles become very weak, patients may have difficulty breathing, which can lead to low oxygen levels or a buildup of carbon dioxide in the blood. In rare cases, the heart muscle can also be affected, leading to cardiac complications.^[15]

With modern treatment approaches that combine immunosuppressive medications, physical therapy, and careful monitoring for complications, the outlook for most patients with necrotizing myositis is much better than it was in the past. Early diagnosis, aggressive treatment, and ongoing medical care are key factors in improving both quality of life and long-term survival.

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